Field of the Invention
This invention relates generally to organic light emitting diodes, solar or photovoltaic (PV) cells, daylighting windows, light extracting substrates, substrates with friction modified surfaces, and methods of making the same.
Technical Considerations
An organic light emitting diode (OLED) is a light-emitting device having an emissive electroluminescent layer incorporating organic compounds. The organic compounds emit light in response to an electric current. Typically, an emissive layer of organic semiconductor material is situated between two electrodes (an anode and a cathode). When electric current is passed between the anode and the cathode, the organic material emits light. OLEDs are used in numerous applications, such as television screens, computer monitors, mobile phones, PDAs, watches, lighting, and various other electronic devices.
OLEDs provide numerous advantages over conventional inorganic devices, such as liquid crystal displays. For example, an OLED can function without the need for a back light. In low ambient light, such as a dark room, an OLED screen can achieve a higher contrast ratio than conventional liquid crystal displays. OLEDs typically are also thinner, lighter, and more flexible than liquid crystal displays and other lighting devices. OLEDs typically also require less energy to operate than many other conventional lighting devices.
However, one disadvantage with OLED devices is that they typically emit less light per unit area than inorganic solid-state based point-light sources. In a typical OLED lighting device, a large percentage of the light emitted from the organic material is trapped inside the device due to the optical waveguide effect in which the light from the organic emitting layer is reflected back from the interface of the organic emitting layer/conductive layer (anode), the interface of the conductive layer (anode)/substrate, and the outer surface/air interface. Only a relatively small percentage of the light emitted from the organic material escapes the optical waveguide effect and is emitted by the device. Therefore, it would be advantageous to provide a device and/or method to extract more light from an OLED device than is possible with conventional methods.
Photovoltaic solar cells are in principle counterparts to light emitting diodes. Here, the semiconductor material absorbs the energy of light (photons) and converts that energy into electricity. Similar to OLEDs, the efficiency of the photovoltaic device is relatively low. At the module level, for example, typically only up to 20% of the incident light is converted to electric energy. In one class of photovoltaic devices, those consisting of thin film PV cells, this efficiency can be much lower, depending on the semiconducting material and the junction design. Therefore, it would be advantageous to increase the fraction of the solar light that is absorbed near the photovoltaic semiconductor junction to increase the efficiency of the photovoltaic device.
OLEDs and photovoltaic devices are typically made in batch coating processes in which each coating layer is applied in a coating station. The substrate is then transferred to another separate coating station for application of the next layer, and so on. This is a time intensive and labor intensive process. It would be advantageous if two or more of the coating layers or functional regions of the device could be made in a continuous process rather than a batch process. It would also be advantageous if the friction coefficient of a substrate could be modified, for example in a continuous coating process.